River Atlas

Welcome to Snohomish County's River Atlas webpage. The River Atlas provides easy access to river analysis, design, and restoration work, and historical data including photographs of Snohomish County's rivers through the ages.

The atlas includes all large rivers in the county, and eliminates the need to search multiple studies and reports for information including river flows and channel migration.

River Atlas

The Atlas

  • Glossary of Terms (PDF)
  • View the River Atlas - Interactive maps for each of the assessments are available with data layers such as:
    • Channel migration (riverbed redirection through time)
    • Orthophotos (enhanced aerial photos)
    • Land parcels
    • Hydraulic model cross sections (showing water flow)
    • "Parker" diagrams (showing areas of sediment buildup and erosion)
    • Avulsion risk ratings (likelihood of river redirection)
    • Potential restoration project locations


Who will want to use this River Atlas? Scientists and river advocates studying different aspects of the river, developers and landowners interested in how rivers may affect their property, and those genuinely interested in learning a bit more about what makes Snohomish County's rivers such a dynamic and rich resource to the county.

Generally speaking, the atlas can be used to:

  • Determine erosion / flooding risk on public and private property.
  • Prepare project designs to the conceptual level (final designs require additional engineering).
  • Extract information for permitting purposes (e.g., restoration, bank stabilization, infrastructure improvements).
  • Determine the potential for restoration projects, and what type might work best.
  • Create an understanding of how Snohomish County's rivers have migrated and changed the surrounding land throughout history.

We will continue updating the atlas to accommodate river changes, especially following significant storm events. Smaller systems, such as the Wallace or Pilchuck rivers, will be added as time and funding allow.

If you have any questions, email SWM GIS Support or call 425-388-3464.


  • All county, large river assessment work completed to date
  • Summary values from habitat inventory work
  • Designs for existing restoration projects
  • Monitoring data associated with restoration projects.

What's Next

  • We intend to update the atlas after selected significant storm events, (e.g., five year events), in order to stay current with this information.
  • Smaller systems, such as the Wallace or the Pilchuck, will be added as time and funding allow.